1. Field of the Invention
The present invention relates to an image forming process using a laser beam.
2. Description of the Prior Art
As is well known, o-quinonediazide compounds are photosensitive substances used in photosensitive printing plates, photoetching resists or photosensitive copying layers for projected originals, etc. It has been believed that the o-quinonediazide compounds form carboxyl group containing compounds by decomposition of the diazo group by application of actinic radiation. Therefore, when a copying layer containing the o-quinonediazide compound is developed with an alkaline developer after exposure to light, the exposed portion is removed from the base to form an image composed of the unexposed portion, i.e., so-called positive-positive type image formation.
In processes carried out hitherto, a photographic printing technique has been used which comprises contact or projection exposure to form a print. In producing an original for photographic plate making using the production of printing plates as an example, it is necessary to carry out a complicated processing. Namely, processing comprises photographing an original print obtained from a typographic composition on a lithographic film by means of a plate making camera, developing and fixing to form an offset printing plate. A so-called facsimile process comprises scanning the original print to transmit by telephone or telex circuit, scanning a photosensitive plate similarly at the receiving end to expose a facsimile film, developing and fixing, or exposing a film for photo-typesetting by means of a letter generation device of a hand-operated photo-typesetting machine or a computer photo-typesetting machine, developing and fixing; or making an original by exposing printing paper for photo-typesetting developing and fixing, exposing a film using the resultant original by means of a plate making camera or a facsimile device, developing and fixing.
As indicated above, in the prior plate making processes, it is necessary to produce a transparent original for plate making by the above described means prior to the plate making. On the other hand, as a recent development in facsimile transmission or in computers, it has been found highly desirable to use output signals in large quantities at a high speed. Such a process comprising printing on a printing plate using a laser beam modulated by the output signal has been proposed.
As the process for recording images by laser beam information without using an original for phototypesetting, there is a process described in Japanese Patent Application (OPI) 102401/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") which comprises forming a metal layer as a laser recording layer by vacuum evaporation of aluminium or copper, etc. on a photosensitive layer such as a PS plate, etc. or bonding a mask produced by vacuum evaporation of zinc on a polycarbonate film, and selectively removing the metal layer using a laser to produce a transparent original, by which exposure for a plate making step can be carried out at once.
However, the above process has the following fatal defects. Namely, the metal layer is formed directly on the photosensitive layer of a known PS plate by vacuum evaporation as described in Example in said patent application and there is the possibility of degradation of the photosensitive layer by light or heat emitted from an evaporation source upon vacuum deposition, which makes mass production nearly impossible.
The second defect is that to avoid the first, a mask produced by vacuum evaporation of zinc on a polycarbonate film is bonded to the photosensitive layer. However, since the above described mask brings the polycarbonate film into contact with the photosensitive layer of the printing plate, the metal layer functioning as a transparent original does not closely contact with the photosensitive layer of the printing plate. Consequently, resolving power deteriorates, because exposure of the printing plate is carried out through the thickness of the film.
A third defect is that high energy is required for writing on the recording layer by laser, because the recording layer is aluminium, copper or zinc. Accordingly, it is not desirable for practical use.
In order to remove the above described defects, it has been proposed in, for example, Japanese Patent Application 59593/78 that the photosensitive layer of the printing plate is produced so as to have sensitivity to visible radiation such as an argon ion laser beam. Namely, an addition-polymerizable compound having at least one ethylenically unsaturated double bond and a merocyanine dye capable of absorbing the argon laser radiation are combined as a part of a photopolymerization initiator, by which the system has the sensitivity to the laser radiation. According to this process, the layer structure and the plate making step are simplified as compared with the process described in Japanese Patent Application (OPI) 102401/75, as a progressive technique. However, this process suffers fog under yellow light, because the sensitizer (photopolymerization initiator) itself is sensitive to visible light such as emitted by an argon laser. Therefore, this process has a large defect in working properties, because it is necessary to handle it under red light. Further, in the system utilizing the above described photopolymerization reaction, it is necessary for obtaining stabilized image quality (1) to provide an air-intercepting layer on the photosensitive layer or (2) to carry out conditioning in a nitrogen gas prior to exposure, because the photopolymerization reaction is remarkably hindered by oxygen in the air. The treatment in case of (2) is very troublesome and the layer structure in case of (1) is complicated.
As a result of studies of processes for sensitizing many organic photosensitive materials to laser radiation in order to remove the above described defects, it has been found that the above described defects can be removed by incorporating in the photosensitive layer a compound which is not sensitive to common indoor yellow illumination but is sensitive to only high energy radiation such as laser radiation, by which the rate of dissolution of a photodecomposition product of the above described o-quinonediazide compound in the developing solution becomes lower than that in an unexposed portion. Further it has been found that negative images are obtained, namely, one in which the laser unexposed portion is removed by dissolution in the developing solution (negative working).